Photoconductive pickup tube having an electrically isolated mesh assembly



Jan. 15, 1963 D. MILLER ETAL 3,073,981 PHOTOCONDUCTIVE PICKUP TUBEHAVING AN ELECTRICALLY ISOLATED MESH ASSEMBLY Filed Aug. 30. 1960 2Sheets-Sheet 1 YINVENTORS lrraAA/Ef w m d Ex u m W D n mm 'wE/W L. D.MILLER ETAL 3,073,981

UP TUBE HAVING AN ELECTRICALLY ISOLATED MESH ASSEMBLY 2 Sheets-Sheet 2Jan. 15, 1963 PHOTOCONDUCTIVE PICK Filed Aug. 30. 1960 I NVENTOR5 LouisDmi'ller' 8 Elvin m musselman :5 @M

ITTJEA/E) Z 7/ R 4 w 4 f Q c c F H. Z /l .v l I n a v i 7 n A? g J i W X8 W k 1 l \X x w a .2 4 2 3 Z f I VV 1 2 %H 3,973,981 PHOTOCONDUCTIVEPIOKUP TUBE HAVING AN ELECTRICALLY ISOLATED MESH ASSEMBLY Louis D.Miller and Elvin M. Musselman, Lancaster, Pa.,

assignors to Radio Corporation of America, a corporation of DelawareFiled Aug. 30, 1960, Ser. No. 52,889 Claims. (Cl. 313-65) This inventionrelates to pickup or camera tubes of the type using a photoconductivematerial as the photosensi tive element. In particular, this inventionrelates to an improved photoconductive pickup tube including a novelmeans for mounting the decelerating mesh screen electrode.

One of the several known types of pickup tubes generally comprises anevacuated tubular envelope enclosing an electron gun and aphotoconductive target electrode. The target electrode is supported upona transparent support member, which normally is the optically cleartransparent face plate sealed to the end of the envelope and facing theelectron gun. The target electrode includes a transparent conductivecoating, or signal electrode, on the gun side of the transparent supportmember and a photoconductor comprising a deposit of photoconductivematerialon the transparent conductive coating. Photoconductive materialsare materials which change their electrical conductivity in response toincident radiations. These materials have a relatively high electricalresistance when in the dark and a relatively high electricalconductivity when exposed to the light or other radiations of a selectedfrequency. Closely spaced from the surface of the photoconductivematerial, on the side thereof that is exposed to the electron beam, is afine mesh screen or beam decelerating mesh electrode.

One of the problems that has limited the resolution of pickup tubes ofthe type briefly described above, is the problem of secondary electronemission from the decelerating mesh electrode. One of the conventionalmethods of operating tubes of this type is the so called low velocitybeam method, wherein the accelerating potential of the electron beam isof the order of 300 volts or less, and the electron beam is deceleratedby the mesh screen as it approaches the photoconductor. Even when usingthis low velocity method of operation, secondary electrons are emittedfrom the decelerating mesh screen. Various attempts have been made inthe past to eliminate this secondary electron emission. These attemptshave included increasing the electron transparency of the deceleratingmesh, and forming the decelerating mesh screen electrode of a materialhaving a low secondary emission factor. However, none of these attempscompletely eliminates the secondary electron emission from thedecelerating mesh screen electrode.

A further problem in the operation of a conventional typephotoconductive camera tube is that there is a certain amount of pictureshading that occurs between the center and edge of the scannedphotoconductive surface. This shading occurs because of the fact thatthe electrons that are deflected from the axis or center of the tubehave a slightly lower velocity than those traveling along the axis ofthe tube. Because of this difference in electron velocity, spuriousvariations or shading of an output signal can at times be observed.

A still further problem encountered in the operation of aphotoconductive type pickup tube is that residual gas atoms remain inthe tube after it is evacuated and these atoms will produce positiveions, when bombarded by the electron beam. The positive ions may tend toflow, in a field free space, toward the target electrode. If thesepositive ions should land on the target electrode a further spurioussignal or noise is produced.

-- conventional. In the conventional photoconductive type It istherefore an object of this invention to provide an improvedphotoconductive type pickup tube.

It is a further object of this invention to providea novelphotoconductive camera tube characterized in its high resolution,freedom from shading effects, and freedom from positive ion bombardmentof the target electrode.

These and other objects are accomplished in accordance with thisinvention by providing a photoconductive camera tube wherein thedecelerating mesh screen electrode is physically supported by andelectrically insulated from a final tubular focusing electrode. Due tothis structure, secondary electrons generated by the electron beam striking the mesh screen electrode can be collected by the mesh screenelectrode. Also, an electron lens can be produced, during tubeoperation, between the focusing electrode and the decelerating meshscreen electrode, to improve the center to edge picture shading. Stillfurther, a potential may be applied to the decelerating mesh screenelectrode to repel positive ions back toward the electron gun where theyare relatively harmless. The particular structure of this invention issuch that it is rigid, can readily be mass produced and is free of anymaterials which would tend to contribute dirt or other contaminants tothe photoconductive target electrode.

The invention will be more clearly understood by reference to theaccompanying two sheets of drawings wherein:

FIG. 1 is a longitudinal sectional view or a photoconductive type ofpickup tube showing one embodiment of this invention;

FIG. 2 is an enlarged fragmentary sectional view of another embodimentof this invention;

FIG. 3 is an enlarged fragmentary sectional view of still anotherembodiment of this invention.

Referring now to FIG. 1 in detail, there is shown a photoconductivepickup tube 10 of the Vidicon type which includes improvements inaccordance with this invention, as will be pointed out hereinafter. Thetube 10 comprises an elongated, evacuated envelope 12 having an electrongun 14 in one end thereof, and positioned substantially on the axis ofthe tube. The electron gun 14 is for the purpose of producing anelectron beam and directing the beam towards a target electrode 16. Thetarget electrode 16 comprises a transparent electrical conductor 18supported upon a transparent end or face plate Zilof the envelope 12. Onthe transparent electrical conductor 18 there is provided aphotoconductive member 22. The photoconductive member is made of amaterial, the resistance of which decreases when exposed to radiantenergy. Many photoconductive materials are known for many differentspectral regions and a conventional material for the visible spectrum isantimony trisulfide, for example. The transparent conductor 18 may beany material which is transparent to the radiations to which thephotoconductor 22 is responsive and by way of example, a conventionalmaterial for the visible range is tin-oxide. The transparent conductor18 is electrically connected to a sealing ring 24. A potential appliedby the sealing ring to the conductor 18 during tubeoperation will causethe conductor to function as an output electrode or signal plate.

The electron gun 14comprises a thermionic cathode 26, a control grid 28,an apertured accelerating electrode 30, and a focusing electrode 32, orG electrode. The parts of the tube that have been described in detailare pickup tube the tubular focusing electrode 32 is electricallyconnected to a fine mesh screen that spans. the

end of the focusing electrode and is electrically connected thereto. 1

In accordance with this invention, a fine mesh. screen 36 is physicallysupported by, but electrically insulated from the focusing electrode 32so that a potential may be applied to mesh screen electrode 36 that isdiiferent .from the potential that is applied to the focusing elecof theelectrode 32 to form an annular lip 35 adjacent to this end of theelectrode 32. Pressed against the annular lip 35 and coaxial around theouter periphery of the G electrode 32, is a ring-like electricallyinsulating member 40. One example of a material which may be used toform the insulating member 40 is glass that is formed to rather accuratedimensions. For example, a glass ring having a diameter of approximately0.060 inch through its solid portion and an overall inside diameter ofabout 0.75 inch, has been found to be suitable as a support member 40.Abutting the ring support member 40 is a flanged lip 42 of anelectrically conductive mesh support ring 44. Secured to the outerperiphery of the mesh support ring 44 is a second mesh support ring 45,which has a free end extending axially beyond the end of the second meshsupport ring 44, the end of which in turn extends axially beyond the oneend of the focusing electrode 32. Spanning the mesh support ring 45 atits free end and aflixed thereto, such as by spot welding, is thedecelerator elec trode mesh screen 36.

Butting against the lip 42 on the mesh support ring 44 is a secondring-like electrical insulating member 48. The ring member 48 may besimilar in size and construction to the ring 40. Pressing the ring 48into firm contact with the lip 42 and pressing the lip 42 into firmcontact with the insulating member 40 and the lip 35 of the focusingcylinder 32 is a bulb spacer 50. The bulb spacer 50, duringmanufacturing, is firmly pressed against the insulating member 48 tofirmly fix the mesh ring in position and then the bulb spacer is fixed,e.g. by spot welding, to the walls of the electrode 32. The innerdiameter of the lip 42 is larger than the outer diameter of theelectrode 32. The thickness of the insulating ring 40 is sutficientlylarge so that the inner edge of the lip 42 cannot touch the electrode32.

It should be noted that the position of the mesh screen 36 is firmlyfixed with respect to the decelerating electrode 32 by the abovedescribed arrangement. It should also be noted that the insulatingmembers 40 and 48 extend around the periphery of the focusing electrode32 so that the mesh screen 36 is rigidly fixed at all points around thetube axis. Also, since the insulating ring 40 is of a substantiallyuniform cross-sectional dimension, which may be rectangular or round asshown, and since the lip 35 and the lip 42 on the mesh support ring 44are both held in position that is substantially normal to the axis j ofthe envelope, the mesh screen 36, is electrically insulated from thedecelerator electrode 32 and is firmly positioned substantially normalto the tube axis. 7

The members 38, 44 and 45 may be made of metallic materials such as anickel-chromium alloy, and have a thickness such as 0.015 inch or less.The spacing be tween the scanned surface of the photoconductor 22 andthe mesh. screen 36 may be adjusted as desired during the manufacture ofthe tube 10. A conventional spacing between the mesh screen 36 and thephotoconductor 22iis .090 inch. I

Connected to the mesh support ring 44 is a lead-in (not shown) whichextends through the stem of the envelope for the purpose of applying apotential during tube operation to the mesh screen 36. During tubeoperation,

assuming that the cathode 26 is at ground potential, typical operatingvoltages'for the tube are as follows: ap-

-proxirnately minus 50 volts for the control grid 28,

approximately 300 volts .for the accelerator grid 30, and

approximately 280 volts for the beam focus electrode 32. With potentialssuch as these, the preferred operation in accordance with this inventionis to apply approximately 305 volts to the decelerator mesh screen 36and approximately 50 volts to the signal plate 18. a

The tube 10, in accordance with this invention, when utilizing thestructure as illustrated, provides improved picture definition ascompared to conventional pickup tubes. One of the reasons for this isthat secondary electrons that are displaced from the mesh screen 36 bythe electron beam from the gun 14 are collected by the mesh screen 36.The vast majority of secondary electrons have a velocity of less thanapproximately 10 volts when using conventional operating potentials.When the mesh screen 36 and the focusing electrode 32 are at the samepotential, the secondary electrons tend to pass through the screen andland on the photoconductor thus causing spurious signals. When usingthis invention, and since the diflerence in potential between thefocusing electrode 32 and the mesh screen 36 is preferably greater thanthe 8 volt energies of the secondary electrons, the secondary electronswill tend to be collected by the mesh screen and an improved picturedefinition will result.

A further advantage of this invention is that improved focus is produceddue to an advantageous electron lens. The electron lens is formed as aresult of the diflerence in potential existing between the focusingelectrode 32 and the mesh screen 36. This electron lens is strongeradjacent to the envelope walls, since the field is more concentrated dueto the close spacing in this region, and therefore, the field is moreeflectiveadjacent to the envelope walls. Because the field is moreeffective in this region, the field tends to focus the electron morenear the envelope walls than near the tube axis. This more concentratedeffect substantially eliminates the problems of shading of the beam.

A still further advantage of this invention is that positive ionsresulting from residual gas which remains in the envelope, and which hasbeen ionized by the beam, will tend to be repelled by the positive meshscreen 36. In the prior art tubes, the positive ions of the residual gastended to drift in what amounted to a field free space formed by theunipotential structure of the mesh screen and the focusing electrode 32.

It should be noted that the electrode support structure in accordancewith this invention is preferably made of glass and metal. The reasonfor this preference is that materials such as ceramics and micas, asWell as many other known materials, tend to contribute dirt or othercontaminants to the tube structure which might fall onto the mesh screenor onto the photoconductor causing a spot in the reproduced picture.Still further, it should be noted that the radial space occupied by themesh screen support ring 44 and by the insulator rings 40 and 48 isrelatively small so that thisinvention can be practical withconventional sized decelerating electrodes 32 and conventional sizedenvelopes 12.

Referring now to FIG. 2, there is shown a fragmentary sectional view ofan embodiment of this invention wherein a mesh screen electrode 56 iselectrically isolated from a tubular decelerating electrode 32 by asingle insulating ring 58 of rectangular cross section. In thisembodiment, a flanged member 60 is spot welded to the G electrode 32,the insulator ring 58 is pressed against the'flanged member 60 and asecond flanged member 62 is then firmly pressed against the insulator 58and secured, e.g. by spot welding, to the focusing electrode 32.

A pair of flanged members 63 and 64 are provided on the outer peripheryof the insulating member 58 which clamp the insulating member 58 and arespot welded together. When this has been done, an inverted cup shapedmesh support member 68 may be spot welded to the flanged members 62 and64, after the support member 68 has been properly positioned withrespect to the o di decelerating electrode 32 and with respect to thephotoconductive target 16.

Referring now to FIG. 3, there is shown an embodiment of this inventionwherein a support ring 70 is positioned partially within and secured tothe end of the focusing electrode 32. The support ring 70 has a dimpledportion 71 so that a pair of electrically insulating rings 72 and 74 maybe spaced apart around the periphery of the support ring 70 and abuttingthis dimpled portion. The decelerating mesh electrode 76 is supported ona mesh support ring 78 which in turn is supported by a funnel shapedsupport ring 80. The support ring 80 is formed in such a manner that theglass ring 72 is pressed by the conical portion of support ring 80against one side of the dimple 71. Also, the support ring 80 has aretainer ring 81 fixed thereto and spaced from the focusing electrode 32and so positioned that the insulator ring 74 is pressed, by the retainerring 80, against the opposite side of the dimple 71. Thus, the mesh 76is securely fixed with respect to the decelerating electrode 32. A bulbspacer 82 is also atfixed to the mesh support ring 78 to position theelectrodes within the envelope 12. In this embodiment, the glass rings72 and 74 may be split but should extend substantially around theenvelope axis to provide nearly complete and substantially uniformsupport. The inclined surfaces on the support 80 and the dimpled surface71 permit the use of slightly smaller diameter insulating rings 72 and74 as compared to the diameter of the rings used in the embodimentsshown in FIGS. 1 and 2.

In all of the embodiments of the electrically isolated mesh screenassembly in accordance with this invention, materials are used which arefree from contaminants that would harmfully affect the photoconductivetarget. Also, it should be noted that the insulating member, orinsulating members, which may be round or rectangular in solid crosssection, are arranged coaxially around the axis of the deceleratingelectrode so that a uniform peripheral support is given to the meshscreen electrode. Thus, the mesh screen electrode is securely fixedaround the circumference of the decelerating electrode so that anymispositioning of one side of the mesh screen electrode, with respect tothe decelerating electrode or the target 16, is prevented. In all of theembodiments of this invention, the decelerating mesh screen is providedwith a separate lead-in so that a potential may be applied thereto,during tube operation, that is different from the potential applied tothe tubular decelerating electrode, and the problems of secondaryemission, bad shading, as well as the harmful efiects of positive ionsare eliminated.

What is claimed is:

1. A photoconductive pickup tube comprising an evacuated envelope, aphotoconductive target in said envelope, an electron gun in saidenvelope for directing an electron beam toward said photoconductivetarget and having an axis, said electron gun terminating adjacent tosaid photoconductive target, a mesh screen electrode positioned betweensaid electron gun and said photoconductive target, said mesh screenelectrode being supported by and electrically insulated from saidelectron gun by means including an electrical insulator extendingsubstantially around said axis of said electron gun.

2. A camera tube comprisingan evacuated envelope having an axis, anelectron gun in one axial end of said envelope for producing an electronbeam, a photosensitive target electrode in the other axial end of saidenvelope, a

mesh screen electrode positioned in the path of said beam andsubstantially normal to said axis, said mesh screen electrode beingsupported by but electrically insulated from a portion of said electrongun by means including an insulating member positioned substantiallycoaxial with said envelope.

3. A photoconductive pickup tube comprising an evacuated envelope, anelectron gun having an axis and disposed in one portion of said envelopefor producing an electron beam, a target electrode in another portion ofsaid envelope and positioned substantially normal to said axis and inthe path of said electron beam, said gun including a first hollowtubular electrode positioned coaxially with said gun and adjacent tosaid target electrode, a mesh screen electrode, and means for physicallysupporting While electrically insulating said mesh screen electrode onsaid hollow tubular electrode, said means comprising a pair ofelectrically insulating members extending substantially coaxially aroundthe axis of said envelope and around said hollow tubular electrode, saidmeans further comprising a support ring for said mesh screen electrode,a portion of said support ring being sandwiched between saidelectrically insulating members.

4. A photoconductive pickup tube comprising an elongated tubularenvelope, an elongated tubular electron gun having an axis and disposedwithin said envelope, a photoconductive target electrode positionedwithin said envelope substantially normal to said axis and adjacent toone end of said electron gun, a mesh screen electrode positioned betweensaid target electrode and said one end of said electron gun, and meansfor physically supporting said mesh screen electrode on a part of saidelectron gun while electrically insulating said mesh screen electrodefrom said part of said electron gun, said means including an annularelectrical insulating member extending around said axis and around saidpart of said electron gun and fixed thereto, and a mesh screen supportring secured to said electrically insulating member.

5. A photoconductive pickup tube comprising an elongated envelope havingan axis, an electron gun in one end portion of said envelope andextending substantially along said axis, a photoconductive targetelectrode in said envelope and substantially normal to said axis, anapertured mesh screen electrode positioned between said electron gun andsaid target electrode and .substantially normal to said axis, means forrigidly retaining said mesh screen electrode on a portion of saidelectron gun while electrically insulating said mesh screen electrodefrom said portion of said electron gun, said means comprising: a portionof said electron gun having a dimple, two electrical insulating memberseach extending substantially around said axis and around said portion,and each of said electrical insulating members being positioned on adifferent side of said dimple, a hollow truncated conical shaped meshscreen support member, the narrowing diameter of said support memberpressing against one of said insulating members, and a retaining ringfixed to the inner surface of said support member and pressing againstthe other one of said insulating members.

References Cited in the file of this patent UNITED STATES PATENTS2,975,313 Jacobs Mar. 14, 1961

1. A PHOTOCONDUCTIVE PICKUP TUBE COMPRISING AN EVACUATED ENVELOPE, APHOTOCONDUCTIVE TARGET IN SAID ENVELOPE, AN ELECTRON GUN IN SAIDENVELOPE FOR DIRECTING AN ELECTRON BEAM TOWARD SAID PHOTOCONDUCTIVETARGET AND HAVING AN AXIS, SAID ELECTRON GUN TERMINATING ADJACENT TOSAID PHOTOCONDUCTIVE TARGET, A MESH SCREEN ELECTRODE POSITIONED BETWEENSAID ELECTRON GUN AND SAID PHOTOCONDUCTIVE TARGET, SAID MESH SCREENELECTRODE BEING SUPPORTED BY AND ELECTRICALLY INSULATED FROM SAIDELECTRON GUN BY MEANS INCLUDING AN ELECTRICAL INSULATOR EXTENDINGSUBSTANTIALLY AROUND SAID AXIS OF SAID ELECTRON GUN.